1. Field of the Invention
This invention relates to the field of composite materials. More particularly, this invention relates to composite materials comprised of polyvinyl chloride reinforced with talc of less than a certain particle size.
2. Description of the Prior Art
Recently, studies have been made on imparting sufficient mechanical strength or thermal resistance to a thermoplastic resin by compounding inorganic fillers. Some resins have been already put to a practical use through this process. Fiberglass has served as a reinforcing material for polyvinyl chloride. Of course, other reinforcing materials, such as asbestos, needle-like calcium silicate (Wollostonite) are known as fillers. These fiber-shaped fillers are able to improve the tensile strength of the matrix unidirectionally along their orientation. However, they lower the tensile strength remarkably in the transverse direction. That is, they make the material anisotropic. Accordingly, these fillers decrease the impact strength of the polyvinyl chloride. Mica is generally known as a reinforcing material of a polymer which anisotropically affects the physical property of a polymer composite material. However, when mica which has a large ratio of length to thickness (i.e., a large aspect ratio) is blended with polyvinyl chloride, the impact strength of this polymer, as well as the tensile strength, is decreased significantly. This may result from the fact that a shearing stress to disperse the mica in polyvinyl chloride incidentally breaks lamellar crystals of the mica, and lowers the tensile strength of the polymer. When this occurs, the portion of the mica which has crystals left unbroken causes further deterioration of the impact strength. Also, the addition of talc prompts the polymer to degradate upon heating, stains it black in the molding process, and further results in spoiling the impact strength.
Generally, it is known that the addition of an inorganic filler hastens the degradation of polyvinyl chloride. The reason for this is believed to be that the iron ion accelerates the removal of hydrogen chloride from polyvinyl chloride. The asbestos-polyvinyl chloride composite material (widely used in the form of a tile) has been studied to improve the thermal stability. According to "The Handbook of the Additives for Plastics and Rubbers" (published by Kagaku Kogyo-sha, 1970), it has been disclosed that dicyanandiamides and melamine are useful in increasing the thermal stability of polyvinyl chloride. However, when only one portion of a dicyandiamide is added to 100 parts of a talc-polyvinyl chloride compound, the thermal stability is decreased. Similarly, adding one part of melamine causes almost no increase in thermal stability. Also, stabilizers such as tribasic lead sulfate or butyl tin maleate do not increase thermal stability of polyvinyl chloride.